- A
R1 has a missing null0 route for the summary, causing it to not advertise the summary correctly.
Why wrong: While a null0 route is recommended for loop prevention, its absence does not prevent the summary from being advertised; it only affects local forwarding.
- B
The summary address 10.1.0.0/22 is configured on the wrong interface; it should be on the interface facing the internal network.
Why wrong: Summarization should be configured on the interface where the summary is advertised, typically the upstream interface, not the internal one.
- C
R1's summary address 10.1.0.0/22 includes the 10.1.1.0/24 subnet, so R1 suppresses the more specific route, and R2 only learns the summary. This is expected behavior, but if the summary does not match the actual network, reachability fails.
EIGRP automatically suppresses more specific routes when a summary is configured. If the summary is correct, this is normal; however, if the summary is too broad or incorrect, specific subnets may be unreachable.
- D
R2 has a route filter that blocks the 10.1.1.0/24 route.
Why wrong: There is no indication of a route filter; the issue is the summary suppressing the specific route.
Quick Answer
The answer is that R1’s summary address 10.1.0.0/22 is suppressing the more specific 10.1.1.0/24 route, causing suboptimal routing and unreachability. This happens because EIGRP summarization on an interface automatically suppresses all component routes that fall within the summary range, so R2 learns only the aggregate and loses the specific path to 10.1.1.0/24. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of EIGRP summary address suppression and the critical need for a matching null0 route on the summarizing router to prevent black holes. A common trap is assuming the summary will always improve reachability, but without verifying that the summary prefix exactly matches the actual network topology, you can inadvertently drop traffic to specific subnets. Memory tip: “Summary suppresses specifics—if the summary is too broad, you’ve built a road to nowhere.”
300-410 EIGRP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of eigrp troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An enterprise network uses EIGRP with route summarization. Router R1 has the following configuration: interface GigabitEthernet0/0 ip summary-address eigrp 100 10.1.0.0 255.255.252.0. Router R2, connected to R1 via GigabitEthernet0/0, shows: 'show ip route eigrp' includes 10.1.0.0/22 but not the more specific route 10.1.1.0/24. Hosts in subnet 10.1.1.0/24 are unreachable from R2. What is the root cause?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
R1's summary address 10.1.0.0/22 includes the 10.1.1.0/24 subnet, so R1 suppresses the more specific route, and R2 only learns the summary. This is expected behavior, but if the summary does not match the actual network, reachability fails.
The summary address 10.1.0.0/22 is being advertised by R1, which suppresses the more specific 10.1.1.0/24 route. This is a common issue when summarization is configured on an interface without proper null0 route or when the summary is too broad, causing loss of reachability to specific subnets.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
R1 has a missing null0 route for the summary, causing it to not advertise the summary correctly.
Why it's wrong here
While a null0 route is recommended for loop prevention, its absence does not prevent the summary from being advertised; it only affects local forwarding.
- ✗
The summary address 10.1.0.0/22 is configured on the wrong interface; it should be on the interface facing the internal network.
Why it's wrong here
Summarization should be configured on the interface where the summary is advertised, typically the upstream interface, not the internal one.
- ✓
R1's summary address 10.1.0.0/22 includes the 10.1.1.0/24 subnet, so R1 suppresses the more specific route, and R2 only learns the summary. This is expected behavior, but if the summary does not match the actual network, reachability fails.
- ✗
R2 has a route filter that blocks the 10.1.1.0/24 route.
Why it's wrong here
There is no indication of a route filter; the issue is the summary suppressing the specific route.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: R1's summary address 10.1.0.0/22 includes the 10.1.1.0/24 subnet, so R1 suppresses the more specific route, and R2 only learns the summary. This is expected behavior, but if the summary does not match the actual network, reachability fails. — The summary address 10.1.0.0/22 is being advertised by R1, which suppresses the more specific 10.1.1.0/24 route. This is a common issue when summarization is configured on an interface without proper null0 route or when the summary is too broad, causing loss of reachability to specific subnets.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 18, 2026
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